This invention relates to the field of electrical resistance heaters, and more particularly to a novel mineral insulated heater having an improved electrical connection for external leads, and a novel method and end adapter useful in the production of such a novel heater.
Mineral insulated, or so-called cartridge, heaters conventionally comprise a resistance element, typically coiled on an insulating core, and a metal sheath that is coaxial with the coil and core and radially spaced from the coil. Filling the space between the sheath and the coil is a mineral insulating material that has an optimum combination of relatively high thermal conductivity and relatively low electrical conductivity. In commercial cartridge heaters, the mineral insulating material is most commonly magnesium oxide. Power is delivered to the heater through lead pins electrically connected to the resistance element and contained within longitudinal bores in the core, each lead pin extending from the core outwardly through an end of the heater. The end terminals of the resistance element extend along the ends of the core and into the longitudinal holes in the core where they are held in electrical contact with the lead pins. Opposite ends of the resistance element are connected to different lead pins so that electrical power can be supplied to the coil. The lead pins are usually solid as opposed to stranded in order to minimize surface area and the detrimental effects of surface oxidation. They are also reasonably rigid so as to be easily assembled. Heaters of this type are described, for example, in Desloge U.S. Pat. No. 2,831,951 and Portmann U.S. Pat. No. 3,839,623.
In conventional cartridge heaters, each internal lead pin extends outwardly from an end of the heater with the outward extension of the lead pin being adapted to be electrically connected to an external lead. Because the internal lead pins are rigid, the electrical connections between the internal lead pins and the external leads occupy a relatively large amount of space at the end of the heater which adds to the "unheated" length of the cartridge heater. Additional "unheated" length is undesirable in that it acts as a heat sink, causes an uneven heat flux and temperature distribution along the length of the heater, reduces the wattage output of the heater in those applications where the length of the heater is limited and prevents use of the heater in certain close quarters applications. Moreover, by being exposed at the end of the heater, such electrical connections are vulnerable to abuse and damage. While enclosures for the connections such as those shown in Portmann have been used, the enclosures add additional "unheated" length to the heater.
The electrical connections between the external leads and the lead pins are typically effected by welding, brazing, or via an upset of clinched mechanical link. None of these means of connection is entirely satisfactory since each is subject to inherent mechanical weakness and, in certain instances, to significant voltage drop and heat generation across the connection. Specifically, the process of making welded or brazed connections adversely heats the constituent materials and, where fluxes are introduced, misuse of these fluxes can cause electrical shorting since the fluxes are conductive. Crimping and other mechanical joints require additional "unheated" length to accomodate the joint.